Method for measuring the magnetic resonance (NMR) by steady state signals (SSFP)

ABSTRACT

A method of magnetic resonance (NMR) for spatially resolved measurement of the distribution of NMR signals of metabolites (CSI) with low signal intensity, wherein on a spin ensemble, a sequence of radio frequency (RF) pulses is applied which are mutually offset by a time interval of a repetition time TR and magnetic gradient fields are switched of which at least one causes spatial encoding of the excited spins, is characterized in that the repetition time TR between the exciting RF pulses is selected to be at the most in the magnitude transverse relaxation time T2* of the spins to be excited, preferably approximately T2*/10 and that the magnetic gradient fields are selected such that their action integral is completely balanced over a repetition period of a time period TR such that NMR signal production is carried out according to the principle of steady state free precession (SSFP). This new method permits utilization of the advantages of SSFP methods also for spectroscopic recordings, in particular for chemical shift imaging.

This application claims Paris Convention priority of DE 101 38 961.2-33filed on Aug. 8, 2001, the entire disclosure of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

The invention concerns a nuclear magnetic resonance (NMR) method forspatially resolved measurement of the distribution of NMR signals ofmetabolites (CSI) with low signal intensity, wherein a sequence of radiofrequency (RE) pulses, which are mutually offset at a time interval of arepetition time TR, is applied to a spin ensemble and magnetic gradientfields are switched, of which at least one causing encoding of theexcited spins.

A method of this type is known e.g. from the publication by T. R. Brownet al. “NMR chemical shift imaging in three dimensions”, Proc. Natl.Avad. Sci. USA, Vol. 79, 3523-3526 (1982).

To measure the spatial distribution of metabolites, one uses todayconventionally the so-called chemical shift imaging (CSI) method whereina signal is recorded through repeating an excitation with thecorresponding excitation pulses which can be carried out with spatialselectivity for selecting a partial volume. The excitation steps arethereby repeated at a time interval TR, wherein TR is within themagnitude of the longitudinal relaxation time T1 of the observedmetabolites to avoid signal saturation. The recording is thereby oftenvery inefficient since the signal decays with the decay constant T2*,wherein T2* is much smaller than TR due to magnetic fieldinhomogeneities such that the actual useful time of data recording isvery small compared to TR.

The method of steady-state free precession (SSFP) (Carr H Y, Phys. Rev.112, 1693 (1958) presented by Carr in 1958 has much more efficientsignal recording in comparison therewith. Therein, application of aregular sequence of radio frequency pulses produces steady statemagnetization which is read out in the time interval between the pulses.The magnetization strength depends on the resonance frequency of theobserved spins. In the preferred implementation, the phase of subsequentpulses is alternated. For spins, which experience dephasing by 180° inthe time interval TR between two pulses, the signal is therebyminimized.

For MR imaging (i.e. measurement of the proton density), this so-calledtrueFISP method (also called balanced FFE or FIESTA) has already beenestablished (Oppelt A et al, electromedica 54, 15 (1986) and is oftenused with new devices since the available rapid gradient systems canachieve repetition times of typically TR<5 ms which are short enough toprevent image artefacts which are produced by the signal cancellation ofspins which are dephased by field inhomogeneities.

There are approaches of optimizing the behavior of magnetization in thetransition phase from the balanced state into the steady state.Initialization with a pulse with half a flip angle is introduced herewhich precedes the following sequence at a time interval of preferablyTR/2 (Deimling M, Heid O. Magnetization prepared true FISP imaging. In:Proceedings of the 2^(nd) Annual Meeting of the Society of MagneticResonance, San Francisco, 1994, p. 495). Also more recent solutions withother preparation phases are known.

For applications in proton imaging, trueFISP is therefore established asmethod for very effective data recording. Applications for localizedspectroscopy with SSFP methods and for measuring the spatialdistribution of metabolites were not yet reported although the smallefficiency of data recording of conventional methods and the associatedlong measuring times are the main problem of in vivo MR spectroscopy.

The reason therefore is the fact that the SSFP signal mechanismprimarily and obviously eliminates spectroscopic information since spinsare refocused independent of their resonance frequency and thereforegive a non-distinguishable contribution to the entire signal and thedifferentiation of spins of different chemical shifts (and thereforedifferent resonance frequencies) desired in spectroscopy is lost.

In contrast thereto, it is the underlying purpose of the presentinvention to improve a method of the above-mentioned type such that theabove-discussed disadvantages can be eliminated. The invention is topresent in particular a new method with the aim that the advantages ofSSFP methods can still be used for spectroscopic recordings and inparticular for chemical shift imaging.

SUMMARY OF THE INVENTION

This object is achieved in accordance with the invention in an effectivefashion in that the repetition time TR between the exciting RF pulses isselected to be in the magnitude of the transverse relaxation time T2* ofthe spins to be excited at the most and that the magnetic gradientfields are selected such that their action integral over a repetitiontime of a length of TR is zero such that NMR signals are producedaccording to the principle of steady state free precession (SSFP).

A drastically reduced repetition time TR compared to conventional CSIrecordings and switching of integrally completely balanced gradientspermits maintenance of the spectroscopic information of chemical shiftalso for SSFP recordings. This permits utilization of the advantages ofSSFP methods also for spectroscopy.

One variant of the inventive method is particularly preferred whereinthe repetition time TR is selected to be between 1 and 100 ms,preferably between 5 and 20 ms. The optimum chosen repetition time Trdepends on the other experimental parameters. The above-mentioned valuesare valid in particular for application of a homogeneous NMR magneticfield B in the magnitude of 1-2 tesla.

The signal recording time TAQ is usually always smaller than therepetition time TR. The signal-to-noise ratio per time unit isparticularly large when the signal recording time TAQ is selected to beslightly smaller than TR, preferably TAQ≦0.95 TR.

In a further development of this method variant, the signal acquisitionis always carried out when no RF pulses are currently irradiated. Inthis fashion, the NMR signal can be optimized with respect to noiseminimization.

In a particularly preferred variant of the inventive method, RF pulsesare irradiated and temporally variable magnetic gradient fields areselected for spatial encoding according to the principle of thespatially resolved Fourier transformation method. This greatlyfacilitates reconstruction of spatially resolved images of themetabolites from the recorded NMR signals.

In a further preferred method variant, switching of a magnetic gradientfield spatially limits the excitation volume simultaneously withirradiation of the exciting RF pulses. This facilitates preciselimitation of the NMR measurement to certain parts of the measuringobject thereby keeping disturbances and noise outside of the zone ofinterest away from the signals.

A further development of this method variant is characterized in thatthe direction and amplitude of the slice selection gradient is variedfrom one recording step to the next, thereby further limiting themeasuring volume to the region in which the SSFP condition is met. Thislimits the interesting measuring volume in several dimensions to permitprecise selection of very special subvolumes in the measuring object.

In a further particularly preferred variant of the inventive method, theNMR recording is repeated several times thereby varying the measuringfrequency such that the signal intensities of several NMR signals ofdifferent resonance frequencies overlap in a characteristic fashionacross the measured signal intensity as a function of the measuringfrequency.

As an alternative or supplement, a further method variant provides thatthe NMR recording is repeated several times thereby varying a phaseincrement between subsequent RF pulses such that the signal intensitiesof several NMR signals of different resonance frequencies overlap in acharacteristic fashion across the measured signal intensity as functionof the phase increment between subsequent RF pulses.

In both method variants (and combinations thereof) the signalintensities associated with the individual resonances can be determinedby calculating methods known per se.

In a further advantageous variant of the inventive method, RF pulseswith alternating flip angle α or phase increments of 180° are selected.Usually, the highest possible signal intensities can be achieved.

One variant of the inventive method is also advantageous wherein RFpulses with a flip angle α are selected such that cosα=(T1/T2−1)/(T1/T2+1) wherein T1 is the longitudinal relaxation time andT2 is the transverse relaxation time without taking into considerationthe susceptibility effects. For metabolites wherein T1 and T2 is known,the NMR signal can be maximized.

Further advantages of the invention can be extracted from thedescription and the drawing. The features mentioned above and below canbe used in accordance with the invention either individually orcollectively in any arbitrary combination. The embodiments shown anddescribed are not to be understood as exhaustive enumeration but ratherhave exemplary character for describing the invention.

The invention is shown in the drawing and explained in more detail bymeans of embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings show in detail:

FIG. 1 a trueFISP method according to prior art: rf characterizes theradio frequency pulses with flip angle α and phase φ and the signalrecorded within the acquisition time TAQ; GS, GR and GP characterize theslice selection, read and phase gradients.

FIG. 2 SSFP method with 2D-CSI encoding in accordance with theinvention:

a) the recording is spectroscopical without gradient, a two-dimensionalspatial encoding is produced by successive variation of GP1 and GP2 inan orthogonal direction each.

b) trueFISP method with 3D-CSI encoding: the recording is carried outspectroscopically without gradient, three-dimensional spatial encodingis carried out by successive variation of GP1, GP2 and GP3 in anorthogonal direction each.

FIG. 3 SSFP methods for selection of a partial volume by means of sliceselection gradient in accordance with the invention. In FIG. 3a), twoorthogonal slices are varied in successive recordings, FIG. 3b) showsfurther limitation through variation of the slice selection gradients inall 3 spatial directions.

FIG. 4 comparison of the recording with SSFP and conventional recordingfor T1=4 s, T2=500 ms, T2*=50 ms, TR=5 s, TAQ for the conventionalrecording=60 ms (corresponds to maximum signal-to-noise). The FID signalIfid is characterized by the thick fully drawn exponential signal decay,the SSFP amplitude Itr corresponds to the thick broken line. Theparameters correspond to the measurement of phosphocreatine in in vivo31P spectroscopy. The efficiency ratio Etf/Efid is calculated at 14:1,with f=TAQ/TR=0.7 for the SSFP acquisition Etf/Efid˜10. The shaded arearepresents the recording time of the FID, with longer TAQ, S/N drops.Afid(TAQ) is the central value of the FID amplitude over TAQ.

FIG. 5 recording with parameters of FIG. 4, however, with TR=2.5 s. Theshort repetition time reduces the signal amplitude Ifid, the efficiencyof the FID recording is still improved by the double number of FIDs pertime unit.

Etf/Efid is ˜8 at f=1.

FIG. 6 contribution of the signal intensity Abs(Itr) of the SSFPsequence as function of the off resonance frequency Ω for flip angleα=30° (top), 60° (center) and 90° (bottom) for different ratios of T2/T1and for recordings with alternating pulse phase. One can see that theshape of Itr(Ω) depends only little on the T2/T1 ratio;

FIG. 7 spectral resolution of the SSFP-CSI recording: the spectralresolution is determined by 1/TAQ, the grid of the spectral recording ismarked by thick dots. The signal intensities measured in each case aredetermined by the dependence on the resonance frequency, i.e. the signalintensities are correspondingly modulated. In contrast to FIG. 6, themodulation function itself is shown and not its contribution to indicatethe periodic inversion of the signals.

FIG. 8 recording of the signals of two metabolites A and B with theresonance frequencies Ω_(A) and Ω_(B). The recording can be carried outsuch that the signal of one metabolite each becomes 0.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The trueFISP method for proton imaging corresponding to prior art isshown in FIG. 1. The radio frequency pulses with flip angle α and phaseφ are designed as selective pulses for recording as slice selectionpulses in connection with the read gradient GS, the phases φ ofsubsequent pulses preferably differ by 180°, e.g. φu (uneven recordingperiod)=0° and φg (even recording period)=180°.

To use the trueFISP method for the recording as 1, 2 or 3-dimensionalchemical shift imaging (CSI)=method, the recording is carried outwithout read gradient and spatial encoding is replaced by phasegradients in 1, 2 or 3 spatial directions. The excitation pulse canthereby be produced in a slice-selective fashion either throughapplication of a corresponding gradient during the pulse (for 2D-CSIencoding in FIG. 2a), (shown for 2D-CSI encoding in FIG. 2a) for 3D-CSIencoding (FIG. 2b), the slice selection can be omitted.

Further limitation of the volume within which the above-mentionedcondition for producing a steady state signal is achieved by alternatingpulse phases in subsequent pulses within a square or cylindrical partialvolume is given when the slice selection is varied in subsequent pulses(FIG. 3a).

In the most simple case, subsequent pulses with alternating phase areapplied to orthogonal slices each. The SSFP condition is then met onlyfor the spins in the sectional volume of the two slices. Thisalternation of the slice plane corresponds formally to a recording withrotation of the slice level from one excitation to the next byΔΦ_(s)=90°. Selection of other values for ΔΦ_(s) corresponding to aslower rotation of the slice level permits selection of cylindricalexcitation volumes with corresponding different shapes. For GS1²+GS2²=1,the thickness of the selected slice is always identical and the selectedvolume becomes isotropic in the direction of GS1 and GS2.

Finally, a cubic or spherical volume can be applied by applying variableslice selection gradients in all three spatial directions (FIG. 3b).GS1²+GS2²+GS3²=1 selects a spherical volume. It is advantageous in thismanner if the length of a recording cycle is an integral under differentgradients which permits the most simple realization of the condition ofphase alternation in the target region.

The position of the volume defined according to one of the describedsteps can finally be positioned anywhere in space via correspondingselection of the excitation pulse frequency. The different possibledefinitions of the volume coverage through slice selection can becombined in any fashion with the kind of spatial encoding correspondingto the FIGS. 2a and 2 b to obtain spatial resolution within the selectedpartial volumes.

The acquisition time TAQ is smaller than TR. TR is generally selected tobe short (in the range of a few ms) to minimize susceptibility-basedphase effects which disturb the SSFP condition. For n1 data points whichare recorded within TAQ, the recording bandwidth BW=n1/TAQ iscorrespondingly large. For encoding a 2-dimensional image with np1 phaseencoding steps in the direction GP1 and np2=n1 steps in the direction ofGP2, np1×np2 recording steps are required corresponding to a totalrecording time Tges of np1×np2×TR.

Observation of the signal-to-noise ratio per time unit of a recordingwith conventional trueFISP with read-out by a read gradientcorresponding to FIG. 1 and a CSI phase encoding (FIG. 2a), shows thatthese are initially identical according to the basic theorems of thesignal theory.

In conventional recording with n1 points, np2 recordings are repeated toobtain the same measuring time of np1×np2×TR of the recording with CSIencoding corresponding to FIG. 2b. Corresponding to BW=n1/TAQ, thesignal-to-noise ratio is reduced compared to the CSI recording by afactor of n1 per recording step, with n1-fold averaging, the factor iscompensated again.

CSI encoding is advantageous when the bandwidth per pixel dwp=1/TAQdetermined by TAQ is larger than the line width of the observedresonances. For typical values for TAQ in the region of a few ms, thebandwidth per pixel is in the range of several hundred Hz compared to aline width of 5-10 Hz determined substantially by magnetic fieldinhomogeneities. The line is therefore smeared across the entire pixelwith spatial encoding with a read gradient (FIG. 1).

For recording through phase encoding (FIG. 2a), the signal intensityhowever is focused by the spectral resolution to the resonance frequencyand therefore all noise contributions outside of the resonance signalare separated which considerably improves the signal-to-noise ratio.

A further advantage of spectral recording results from the signalintensity as function of the off resonance frequency. Due to the finitelength of the pulses and phase encoding gradients, TAQ=f*TR with f<1.The spectral resolution SW of the data recording results for complexsignals in SW=1/TAQ according to the Nqyuist theorem. Correspondingly,signals whose resonance frequency differs by 1/TAQ can be recordedseparately and their distribution can be determined separately.

As shown in FIGS. 2a and 2 b, the method can be applied as CSI methodfor measuring the total intensity of all signals of the spin speciesobserved. This is generally not sensible for conventional proton imagingon the basis of the dominant signal portions of water and fat since forhigh-resolution images of an approximate matrix size of 128×128 evenwith TR=2-4 ms, the total recording time Tges is in the region of oneminute and a conventional trueFISP experiment provides a sufficientsignal-to-noise ratio already with n1-fold faster recording.

For CSI applications for observing metabolite signals and in particularsignals of other nuclei, the CSI recording can be utilized in a usefulfashion. A preferred application is in phosphor spectroscopy which isused in in vivo applications mainly to examine the energy metabolism andthe phosphorester metabolism. Of the resonances observed in the in vivospectrum, in particular the signals of phosphomonoesters and thephosphocreatin are suited for SSFP observation since they have arelatively favorable T2/T1 ratio.

The efficiency of data recording compared to conventional recordingthrough CSI with long TR is compared below:

If T2* effects can be neglected and for TR<T1, T2, the signal intensityItf of an SSFP recording for on-resonance spins is given by

Itf=I0*sin(α)/(1+T2/T1+cos(α)*(1−T2/T1))  [1]

wherein I0 represents the equilibrium signal given by the spin density,α is the flip angle of the pulses. Equation [1] shows that I becomeslarge in particular for large values of T2/T1. Due to T2<T1, the maximumvalue for I is obtained at T2=T1.

The recording by SSFP-CSI therefore represents the possibility ofspatially resolved selective spectroscopic observation of narrowresonances (relatively long T2).

To compare the efficiency of the recording with conventionalspectroscopic recording technology, the intensity given by equation [1]must be set into relation with the intensity with multiple repetition ofthe recording of a free induction decay (FID) as it is applied inspectroscopy. For a repetition time TR and a decay time T2*, theintensity Ifid is given by

Ifid=Iss exp(−t/T2*)  [2]

Iss is thereby the steady state intensity corresponding to

Iss=I0 (1−exp(−TR/T1))  [3]

The signal amplitude Ass results through integration via Ifid via therecording time TAQ from equations [2] and [3]:

Ass=I0 T2*(1−exp(−TR/T1))(1−exp(−TAQ/T2*))  [4]

The signal-to-noise ratio is calculated

Sfid=Ass/TAQ.  [5]

Sfid initially increases with increasing TAQ, with long TAQ, itdecreases again since the signal which decays with T2* becomes smallcompared to constant noise. It can be shown that the maximum averagesignal amplitude is approximately 57% of Iss. Averaged over the entirerecording time, one obtains from [2]-[5] an average signal yield Efid ofthe recording of

Efid=Sfid*TAQ/TR=I0T2*(1−exp(−TR/T1))(1−exp(−TAQ/T2*))TAQ/TR  [6]

If the acquisition time of the SSFP recording is selected to be so shortthat T2* effects can be neglected, the corresponding signal yield of thetrueFISP signal results with acquisition over the same period TRcorresponding to equation [1] in:

Etf=fItfTR=fI0*sin(α)/(1+T2/T1+cos(α)*(1−T2/T1))TR  [7]

f is thereby a factor stating which portion of the entire measuringperiod is used for data acquisition in trueFISP recording. For veryshort pulse sequences, f 0.5 since then the duration of the pulses andof the phase encoding gradients becomes similar to TAQ. For longerintervals f˜1.

A comparison of equation [6] and equation [7] shows that the two methodsdepend in a very different fashion on the physical parameters T1, T2 andT2*. FIG. 4 shows that, for recording parameters which are typical formetabolite spectroscopy, the signal yield of the trueFISP recording ismore than 10 times larger than with conventional spectrum acquisition!This corresponds to a reduction of the measuring time by more than afactor of 100 to obtain the same signal-to-noise ratio.

A shift in efficiency occurs when T1 of the examined metabolite is shortand correspondingly short repetition times are selected for datarecording (FIG. 5). When very short repetition times are used inconventional recording technology, the signal yield Efid is increased,however, these parameters favor mainly signals of spins with short T1which is true in the metabolite spectroscopy for molecules of averagesize (1000-10000 atomic units) which are associated to unspecificsubstances and whose measurement is often undesired. TrueFISP thereforeproduces a high signal yield mainly for signals with narrow lines.

Recording with trueFISP therefore produces a considerable signal gaincompared to conventional spectroscopy compared to conventional recordingmainly for metabolites with relatively long T2 (sharp lines). Equations[6] and [7] clearly show that trueFISP is particularly advantageous whenT2*<T2, i.e. for observing signals of small metabolites with long T2.

The preferred application therefore refers to the observation of signalswith T2>T2*, i.e. the line width is determined by magnetic fieldinhomogeneities and susceptibility effects and not by T2. Moreover, itmust be stated that the above calculation neglects T2* effects over theacquisition time of the SSFP recording, i.e. TAQ<T2*.

The different metabolites can be differentiated corresponding to theprinciple of chemical shift imaging via Fourier transformation of therecorded steady state signal. Corresponding to the Nyquist theorem, thespectral resolution is thereby dw=1/TAQ, the bandwidth of the recordingresults for n1*dw. In the border case which is practically notrealizable, when the duration of the radio frequency pulses and thephase encoding gradients is neglected, TAQmax=TR and thereforedwmax=1/TR. In contrast to conventional recording, it must be taken intoconsideration that the signal amplitude is modulated corresponding tothe dependence on Ω shown in FIG. 6. For TAQ<TR, one obtains the imagescreen shown in FIG. 7. To optimize S/N, the SSFP recording is carriedout typically with a repetition time in the region of 1-50 ms, dw istherefore in the region of approximately 20-1000 Hz. This shows clearlythat SSFP-CSI has a considerably worse spectral resolution thanconventional CSI.

Improvement of the spectral resolution is possible through thedependence of the signal intensity on the off resonance frequency. Ifthe recording is repeated with different recording frequency, thiscorresponds to a shift of the recording screen shown in FIG. 7 comparedto SSFP modulation.

Change of the respective carrier frequency by ΔΩ can be calculatedwithout any problem into a phase increment of the phase of subsequentpulses. Therefore, the signal behavior of a recording with a frequencyshifted by ΔΩ compared to the resonance frequency with identical pulsephase is identical to a recording on-resonance (ΔΩ=0) but with a linearphase increment ΔΦ (in radians) corresponding to

ΔΦ=2πTR/ΔΩ  [9]

As shown in FIG. 6, one obtains in particular for an off-resonancefrequency

Ω0=1/(2TR)  [10]

signal cancellation with alternating pulse phase. This corresponds toΩ=0 (on resonance) for a constant pulse phase. The position of theresonance frequencies can be correspondingly determined via the signalmodulation as function of the recording frequency (or of the phaseincrement).

Recording at selected measuring frequencies which correspond to signalsof interest or also by successive recording via a measuring frequencyregion of interest, the intensities of the respective individual signalscan be determined correspondingly.

To record chemical shift selective images of the distribution ofmetabolites with known resonance frequency, in a preferredimplementation the recording can be carried out such that recordingtakes place in several recording steps such that in each individualrecording one signal each is suppressed or minimized (FIG. 8). Themeasured signal intensity represents the sum of the intensities of therespective other signals within the resolution region.

If the number and exact position of the lines of the spectrum to beobserved is not known, the recording is carried out such thatcorresponding selection of the recording conditions measures the signalintensity as function of Ω. The position, intensity and number of theindividual lines of the spectrum can then be calculated viacorresponding algorithms as linear superposition of individual signaldependencies each.

The signal contributions of individual resonances can then be determinedthrough solution of the resulting equation system corresponding to thesuperposition of the contributions of the individual signals accordingto one of the current methods for solving linear equation systems(regression, Marquardt algorithm etc.).

Finally, signals of undesired resonances (e.g. fat and/or water signalsfor proton-CSI) can be suppressed according to prior art in that therecording is carried out such that the modulation function correspondingto FIG. 7 for these signals is at zero passage. Moreover, during thesequence, corresponding radio frequency pulses can be applied foradditional suppression of these signals.

We claim:
 1. A method of magnetic resonance (NMR) for spatially resolvedmeasurement of the distribution of NMR signals of metabolites (CSI) withlow signal intensity, the method comprising the steps of: applying, to aspin ensemble for causing excited spins, a sequences of radio frequency(RF) pulses which are mutually offset at a temporal interval of arepetition time (TR); spatially encoding the excited spins byapplication and switching of magnetic gradient fields; selecting therepetition time (TR) between the exciting RE pulses to be at most atransverse relaxation time T2* of the excited spins; selecting themagnetic gradient fields in order to provide an action integral whichproduces zero over a temporal length between repetition times (TR) suchthat NMR signals are generated according to the principle of steadystate free precession (=SSFP); and recording the NMR signals during asignal recording time TAQ.
 2. The method according to claim 1, whereinthe repetition time TR is selected to be between 1 and 100 ms.
 3. Themethod according to claim 1, wherein the repetition time TR is selectedto be between 5 and 20 ms.
 4. The method according to claim 1, whereinthe signal recording time TAQ is selected to be <≈TR.
 5. The methodaccording to claim 1, wherein the signal recording time TAQ is selectedto be ≦0.95TR.
 6. The method according to claim 4, wherein the NMRsignal acquisition is carried out at time when no RE pulses are applied.7. The method according to claim 1 wherein the RF pulses and magneticgradient fields are temporally varied in order to spatially encode theexcited spins according to the principle of the spatially resolvedForuier transformation method.
 8. The method according to claim 1further comprising the step of limiting an excitation volume byswitching the magnetic gradient field simultaneously with theapplication of exciting RE pulses.
 9. The method according to claim 8,wherein a direction and amplitude of a slice selection gradient ischanged from one recording step to a next in order to further limit theexcitation volume to a region in which the SSFP condition is met. 10.The method according to claim 1, further comprising the step ofrepeating NMR recording thereby varying a measuring frequency such thatsignal intensities of several NMR signals of different resonancefrequencies are overlaid in a characteristic manner as a function of themeasuring frequency over the measured signal intensities.
 11. The methodaccording to claim 1, further comprising the step of repeating NMRrecording thereby varying a phase increment between subsequent RF pulsessuch that signal intensities of several NMR signals of differentresonance frequency are overlaid in a characteristic fashion over themeasured signal intensities as function of a phase increment betweensubsequent RF pulses.
 12. The method according to claim 1, wherein FRpulses with alternating flip angle a or phase increments of 180° areselected.
 13. The method according to claim 1, wherein RF pulses with aflip angle α are selected, such that cos α=(T1/T2−1)/(T1/T2+1), whereinT1 is a longitudinal relaxation time and T2 is the transverse relaxationtime without taking into consideration susceptibility effects.